How does nitroglycerin end up decreasing the workload on the heart, thereby decreasing its demand for oxygen, thereby decreasing a person's chances of experiencing pain (angina pectoris) due to oxygen deprivation?
I. It dilates the veins, which decreases the amount of blood that enters the heart as the heart fills with blood, thereby reducing the tension/pressure on the heart, thereby reducing the demand for oxygen.
II. It dilates the arteries, which decreases the pressure against which the heart must pump. Since this pressure is reduced, the demand for oxygen is decreased.
III. It constricts the arteries, which helps improve the way by which the heart is able to pump blood out into the body.